The present invention relates to a force detector designed for detection of a stress, and more particularly to a detector for converting a force into an electric signal by the use of a piezoelectric element.
In general, force is classified broadly as either dynamic or static. Instruments known for measuring dynamic force include vibrometer, accelerometer, wave-height meter, shock tester, vortex flowmeter and gyro, while those for measuring static force include manometer, differential pressure gauge and load cell (scale).
The force detector according to the present invention is suited to serve as a sensor for use in such measuring instruments.
FIG. 1 shows the structure of a conventional pressure sensor equipped with a piezoelectric element as used heretofore. In the drawing, labeled as 1 is a vessel consisting of a cylindrical part 11, a pressure receiving plate 12 and a bottom plate 13. The pressure receiving plate is shaped into a flexible thin disk with its circumferential end being fixed to the cylindrical part 11, and receives a pressure to be measured. Labeled as 2 is a disk-like piezoelectric unit of which the surface a is bonded to the pressure receiving plate 12 with a bonding agent. When a pressure P to be measured is applied in the above structure, the pressure receiving plate 12 is bent to exert a force on the piezoelectric unit 2 in the direction indicated by an arrow X, so that an electric output corresponding to the flexure of the pressure receiving plate 12 is obtained from the piezoelectric unit 2. To enhance the sensitivity in such a device, it is necessary to widen the pressure receiving plate 12 to increase the quantity of flexure. However, a sensor of intrinsically stout construction is not attainable.
FIG. 2 shows another example of a conventional pressure sensor (disclosed in the Japanese utility Model No. 945397). In this example, a set screw 131 composed of an electric insulator is provided on a bottom plate 13 of a vessel 1, and a piezoelectric unit 2 is supported with a setting pressure applied in the direction indicated by an arrow Y. In this structure, a pressure P to be measured is receivable directly as a force exerted in the same direction since the piezoelectric unit 2 is supported by the set screw 131. However, such supporting by the set screw 131 also brings about a disadvantage that the electric output characteristic of the piezoelectric unit 2 changes widely depending on variation in the support point condition or looseness of the set screw.
In the foregoing examples, a bonding agent composed of, for instance, epoxide resin is used between the joint surfaces a of the pressure receiving plate 12 and the piezoelectric unit 2, but such a bonding agent is not suitable in case the sensor is operated in a high-temperature atmosphere. Moreover, in order to achieve transmission of force, it is necessary that the joint surfaces a of pressure receiving plate 12 and piezoelectric unit 2 are kept in intimate contact with each other. For high temperature use, generally the piezoelectric unit 2 is brazed to the pressure receiving plate 12. In this case, since it is impossible to adopt direct brazing of the piezolectric element, the process is carried out by first forming a platinum layer, which also serves as an electrode, on the joint surface of the piezoelectric unit 2 and then joining it firmly to the pressure receiving plate 12 while utilizing the platinum layer. For the reason that formation of a platinum layer by the art of evaporation is difficult due to inertness of platinum, sputtering is usually adopted to form a platinum layer on the joint surface of the piezoelectric unit 2, hence complicating the operation with resultant increase of production cost. Furthermore, as electric insulation is not attainable between the platinum layer and the pressure receiving plate 12, there exists another disadvantage that a step for ensuring insulation becomes necessary in case such insulation is required.